Horse chestnut leaf miners

The leaf miner, Cameraria ohridella, is a tiny
brown moth only 6-8mm in wingspan. It lays eggs on the leaves of
horse chestnuts and the caterpillars make mines in the leaf tissue
about 1-2mm long. This sounds a very weak threat to the towering
and majestic trees that are not starting to develop the familiar
conkers, but Cameraria soon develops huge populations in
the early summer. The feeding of the caterpillars turns the leaves
brown in mid-season and they drop. The stress on the trees may kill
them but at best the trees become extremely unsightly.

The origin of Cameraria is obscure. It was discovered
and described as a new species in Macedonia in 1986, where a mass
outbreak was found around Lake Ohrid. Then, in 1989, the first leaf
mines were found in Linz, Austria. The following years saw a rapid
invasion into surrounding countries. The pest now covers Germany,
Poland, the Czech Republic, Slovakia, Hungary and has also been
found in Eastern France, Italy, Switzerland, the Netherlands and
Belgium. From the original site in Macedonia it has also spread to
neighbouring Albania, Bulgaria and Romania.

How can such a spread occur?

Horse chestnuts are indigenous to the Balkan Peninsula, although
they have been spread to many parts of the Western world by the
Romans and others as ornamental and shade trees. We can only guess
why cameraria suddenly became a pest: is this a mutant
species? Was it introduced from a more distant location in Asia or
the USA, where it perhaps feeds on a relative of the horse
chestnut?

Certainly the scarcity of parasites in its new environments and
its ability to reproduce more or less continuously from April to
November mean that populations can increase a thousand-fold in one
year. The main suspicion is that the insect is carried around in
the summer by cars, lorries and trains. If you park your car with
the windows open near infested horse chestnuts, moths can find
their way in. They are such poor fliers that they are unable to
escape from a vehicle as larger insects can. Hence, they can be
transported long distances and a car may then later be parked in
the shade – underneath yet more horse chestnuts.

The threat to Britain and the south-east in particular has been
increasing yearly. Insects were detected just outside Paris in
2000, and the intense traffic flow across the English Channel is
likely to support a UK invasion. The first confirmation of the leaf
miners’ presence in the UK was made in July 2002 in Wimbledon,
south-west London. It has now spread north of the Humber and west
of the Severn.